Servo - January 2014

Using passive components is both cheaper and simpler than using level translator chips or MOSFETs.

The fly in the ointment, however, is what resistor values work best for both input and output?

Initially, I grabbed a 15K SIP and eight 8.2K resistors. It worked well for the servos and reading the simple digital inputs. Little did I know the problems I'd run into later ...

I ended up using a 4.7K SIP (RN1) and 2.4K resistors (R1-R8) for the voltage dividers, with a 470 ohm resistor (R9) for the LED. I also added a 100 µF electrolytic capacitor (C1).

You may need to change these values depending on your application. For example, if you want to drive LEDs, using a 910 ohm SIP with 470 ohm discrete resistors would be a good choice.

Decision #2: Choosing between servo, gear, and stepper motors.

I ended up using some generic continous rotation servos I had in my lab — Futaba, Parallax, and GWS are also good readily available choices. I added a standard Parallax servo for panning the range sensor.

There is a wide range of servos on the market — just make sure whichever continous rotation servo you buy has a "centering" potentiometer so you can set the center position of the servo. You can compensate for servos without centering potentiometers in software, but it is simpler if you don't do it this way.

While servos are slightly more expensive than gear motors, we avoid having to build or buy a motor driver which is important in order to minimize the assembly work required before being able to use a functioning robot.

Stepper motors would require an external controller or more I/Os and a driver chip. I wanted to avoid the complexity that using stepper motors would introduce.

I used yellow Solarbotics wheels with the drive servos, so in the future I can add my SirMorph sensors as wheel encoders. Besides, I like how the yellow wheels look with the blue chassis.